The present invention is related to starter-generators, and in particular to a starter-generator that utilizes a permanent magnet exciter.
A generator converts mechanical energy to electrical energy. A motor converts electrical energy to mechanical energy. From a mechanical standpoint, the main difference between a generator and a motor is the direction in which energy flows. Applications that require motoring operations and generating operations, an integrated motor-generator can meet both requirements via a single device. The motor-generator is used in a motoring mode to convert electrical energy to mechanical energy, and in a generating mode to convert mechanical energy to electrical energy. In aircraft applications, a motor referred to as a starter is required to provide mechanical force to the engines for starting. Thus, in aircraft applications, the motor-generator is commonly referred to as a starter-generator.
A typical integrated starter-generator (ISG) includes a main wound-field synchronous machine, an exciter, and a sub-exciter. During operation in the generating mode, the sub-exciter generates electric power that is provided as excitation to the exciter. Typically, the sub-exciter is a permanent magnet generator utilizing a permanent magnet rotor and three-phase stator. When rotating, the permanent magnet rotor induces an alternating current voltage on the three-phase stator that is rectified and provided as a direct current (DC) voltage to the exciter, causing current to build up in the stationary exciter field winding. The current through the exciter field winding induces an AC voltage on the rotating exciter armature windings. The AC voltage is rectified and provided as a DC voltage to a rotating main field winding, which causes current to build through the main field winding. The current through the main field winding induces an AC voltage on the main armature windings of the ISG. The voltage provided on the main armature windings of the ISG is regulated by controlling the current supplied to the exciter field winding. Increasing the current through the exciter field winding increases the generator output voltage, while decreasing the current through the exciter field winding decreases the generator output voltage.
During the motor mode, rather than generate AC voltage at the main armature winding, an AC voltage is applied to the main armature winding. The interaction between the magnetic field created by the main armature winding and the magnetic field created by an energized main field winding results in mechanical energy (in the form of rotation) being applied to the rotating portion of the ISG.